JPH07104552B2 - Lighting device for over head projector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an overhead projector and, in particular, to such a lighting device used in a projector.

Technical background A typical overhead projector is Fresnel
A box-like case containing an illuminating device under the lens, an adjacent pedestal for supporting the trapene, and a projection head adjustably mounted above the pedestal for directing the projected image and It contains a projection head that can be tied up. The illuminator contains an optical device and a light source, usually a tungsten filament lamp. The optic collects as much light as possible from a light source and disperses this light onto a Fresnel lens.

A concave reflector is often installed behind the lamp filament to utilize the light emitted from the back side of the light source. In its most common form, the reflector is spherical and the lamp filament is at the center of the curvature of the reflector, and therefore the inverted image of the lamp filament is at a single magnification and in the same focal plane as the lamp filament. Occurs. When the tungsten filament is constructed with a relatively large clearance, i.e., in a separately wound manner, the position of the reflected image is normally adjusted laterally to occupy the gap in the filament structure. . If the tungsten filament is in a closely-spaced or tightly wound configuration, the filament image is a real filament image in order to eliminate the interference of the reflected filament image by the actual filament again. Can be relocated laterally on one side. In both cases, the screen illumination is increased by the use of a concave reflector.

US Pat. No. 3,979,16 assigned to the assignee of the present invention
No. 0 describes an improved illuminator that achieves improved illumination efficiency with an ellipsoidal reflector and a light source spaced from the focus. The filament image is vertically separated from the actual filament to the extent that the filament image is the only effective light source. It is claimed that with this luminaire, 2000 screen lumens can be achieved with 360 Watt lamps.

DISCLOSURE OF THE INVENTION The present invention provides a lighting device that achieves increased efficiency such that 275 Watt lamps can achieve their goals beyond 2000 screen lumens. Illumination is further significantly increased near the edges and corners of the screen, resulting in an improved corner-to-center ratio over conventional overhead projectors.

Such an increase in efficiency and uniformity is achieved by providing a glass condenser lens, a closely-spaced tungsten filament light source, and a spherical concave mirror. The reflector is positioned with respect to the source filament so that the filament images an axis in the region between the actual filament and the condenser lens. With this restrained vertical separation, the separation from the focal plane of the reflector gives rise to a parasitic condition between the filament and its image. This parametric state provides a lateral separation that increases as the off-axis angular radiation is increased, providing a separation between the filament and its image. Due to the control of this parax, the filament image extends from near-ambiguity on-axis to complete definition at the maximum emission angle of the device. At the maximum emission angle, the filament and its image are a single light source, giving more light to the outside of the screen, resulting in improved efficiency and uniformity, resulting in the efficiency and uniformity previously described. It looks like a single source of light.

The present invention will be described more fully with reference to the accompanying drawings, in which like numerals indicate like parts in several figures.

DETAILED DESCRIPTION Overhead projectors embodying the present invention are illustrated in FIGS. 1 and 2 and are generally numbered.
It is indicated by 10. Projector 10 is the base 12
The lighting device 14, the plane mirror 16, and the Fresnel
Lens 18 and. The foundation 12 of the overhead projector bears a transparent base 20 and a projection head 22 by means of a bearing arm 24. The transparent table 20 supports a trapen (not shown).

The projector 10 is illuminated by the illuminator 14 with a magnified image of this trapene, reflected by a plane mirror 16, collected by a Fresnel lens 18 and projected by a projection head 22. Light directed onto the screen causes it to tie onto a distant vertical surface or screen (not shown). It is preferred in such an overhead projector 10 to provide a lighting device that produces as much useful light as possible at the screen and distributes this light over the entire surface of the trapen. The subject of the invention is a lighting device 14, which is diagrammatically illustrated in greater detail in FIG.

The illumination device 14 includes a light source 26, a concave reflecting mirror 28, and a condenser lens 30. The light source 26 is a tungsten filament which is suitably and conventionally enclosed in a glass bulb (not shown) which is evacuated or evacuated. The filament, as illustrated in FIG. 3, is a structure that is closely or tightly wound and is designated as an FNT type filament by the American National Standards Institute. preferable. Such filament 26
However, molds having a relatively large clearance, that is, wound apart, may be used.

Filament 26 is a slightly magnified view of the filament 34
Are positioned along the illuminator 14 so that they are formed between filament 26 and condenser lens 30. The filament image is illustrated by the image lines in FIG. Concave reflector to form filament image with minimum spherical aberration
The 28 bends should be ellipsoidal. However,
Due to the small longitudinal separation between filament 26 and its image 34, the curvature can be approximated by spherical reflector 28, which is easier and less expensive to obtain. The reflector 28
Is a conventional structure and contains a "cold mirror dielectric coating" that reflects visible light and passes near infrared light to reduce the reflected heat load on filament 26.

The condenser lens 30 is preferably glass to better withstand the heat produced by filament 26, and is more preferably plano-convex (as shown in FIG. 4) or uneven. Well, and is designed to redirect the light produced by filament 26 to cover Fresnel lens 18 adjacent transparent base 20. In this way all the light striking the Fresnel lens can be effectively used to illuminate the trapene and project the image of that trapene on the screen via the projection head 22.

To use the spherical concave mirror 28, as desired,
It is necessary to position the filament 26 inward from the center of curvature of the reflector 28 by a specific distance. The mathematical relationships describing this localization of filament 26 are given below with the letters and symbols corresponding to those shown in FIG.

R = 2F (1) S '= FS / (S-F) (2) M = S' / S = H '/ H (3) D = S'-S = (H' + H) / tan (θ) (4) δ = RS (5) Here, F = focal length of the reflector R = radius of curvature of the reflector S = distance of filament from the vertex of the reflector S ′ = filament image from the vertex of the reflector H = Filament semi-diagonal height H '= Filament image semi-diagonal height M = Filament image magnification D = Filament-filament image separation θ = Reflector helix angle φ = Reflector aperture α = Radiation angle of the light source δ = Separation distance between the filament and the center of curvature of the reflector d = Separation distance between the filament and the glass concentrator The helicopter angle of the reflection mirror, the radiation emitted by the reflection of θ is required. The longitudinal separation, δ, given is simply given by δ = H / tan (θ) (6).

Satisfaction of equation (6) is satisfied at the helicopter angle of the reflecting mirror, θ, and for the light radiated in the directions of the radius of curvature of the reflecting mirror, R, the actual top of the filament 26 is the filament image 34. Makes sure it looks like it touches the bottom. In fact, the lamp filament 26 from the filament statue 34
Is close to 4mm.

The improvement achieved by the particular arrangement of filament 26 with respect to spherical mirror 28 is two-part. In FIG. 1, the illumination reduction with the emission angle, α, persists in both the top and bottom halves of the reflector 28. Second, and more importantly, the separation between the focal plane of filament 26 and filament image 34 causes more of filament image 34 to become visible as the emission angle, α, increases. This is the parallax that was noticed sooner than that obtained by the use of prior art illuminators and when compared to the central illumination level of the Fresnel lens 18, the outer portion of the Fresnel lens 18 Allow more light to be directed.

In theory, a filament in the projection pupil of the projection head 22
The view of filament image 34 and filament image 34 should show that filament image 34 is completely visible and that it is in close contact with actual filament 26 at the end of the exit pupil of projection head 22. 5 to 7 show the image of filament 26 and the filament image 34 formed by Fresnel lens 18 at an angle of 0 ° (on axis,
At the angle between 0 ° and the helicopter angle θ of the reflector (Fig. 6), and at the helicopter angle θ of the reflector (Fig. 7).
Fig.) Shows the filament image 34 as seen in the exit pupil of the projection head 22.

As shown in FIG. 5, at a slight magnification of the filament image, usually about 1.2 times, most of the filament image 34 light is blocked, but some contribution light is found from the filament image 34 along the optical axis. Pay. However, as the off-axis field of view increases, the filament image 34 becomes less obscured and contributes more to the screen illumination.
Near the helicopter angle of the reflector, θ, the filament image should contribute significantly to the screen illumination. If the separation between filament 26 and center of curvature 36 of reflector 28 is chosen to be too small, a partial obstruction of filament image 34 will be near the end of the exit pupil of projection head 22 and if If the separation is too large, filament image 34 will have passed past the exit pupil of projection head 22 near helicopter angle θ, but both such structures are undesirable.

In the region near the helix angle θ of the reflector 28, every ray from the actual filament 26 must contribute to the filament image, or in other words, the filament image should be fully visible. is there. Expressed in another way, the ratio R divided by φ of the reflector 28 should not exceed a value of about 0.56.

EXAMPLE As a specific example of the improvement in illuminance criteria and uniformity that can be achieved within the present invention, measurements were performed for screen lumens and screen edge-to-center ratios at 0.73 and 0.96 of total field. . The spherical concave reflecting mirror 28 has a radius of curvature of 23.9 mm, a caliber φ of 44.0 mm, and 67 °.
Reflector FNT type with a helicopter angle θ of 24 Volts, 275
Used with Watt's lamp. The standard dimensions for this lamp are about 3.5 mm high x 7.0 mm wide, providing a semi-diagonal height H of about 4 mm. The glass condensing lens 30 had a focal length of 44.3 and was of the plano-aspherical type with an aspherical surface close to a parabola. This lens 30 is a lamp
There is a gap between filament 26 and 9mm. Fresnel lens 18 focal length 182.5mm 18 doublet and A4 format (2
The 85 x 285) stage was a doublet projection lens with a focal length of 355 mm with a separation distance used with a projection lens operating at 5.35 times magnification.

Table 1 shows the screen lumen and screen uniformity measured as a function of the lamp filament distance and the distance δ inside the center of curvature of the reflector. The value in brackets is the near-optimal value of δ at which both the 0.96 field screen lumen and the corner-to-center illumination ratio are maximized. This δ value is close to the value predicted by equation (6). Due to this, the efficiency of this lighting system
With a 275 Watt lamp, it is possible to achieve goals beyond 2000 screen lumens. The illumination level was significantly further increased near the edges and corners of the screen, resulting in improved corner-to-center ratios over conventional overhead projector systems.

[Brief description of drawings]

FIG. 1 is a perspective view of an overhead projector which embodies the improved luminaire of the present invention, and FIG. 2 is a cross-sectional elevation view of the overhead projector of FIG. 3 is a perspective view of a tungsten filament preferably used in the present invention, FIG. 4 is a cross-sectional view of the illuminating device, and FIGS. 5, 6 and 7 are images of the filament. 2 illustrates filament images seen at various angles in the exit pupil of the projection lens forming part of the overhead projector of FIG. In the figure, 10 ... Overhead projector according to the present invention, 14 ... Illumination device which is the main subject of the present invention, 26 ... Incandescent bulb filament, 28 ... Truncated concave reflecting mirror, 30 ... Condensing lens , 32 …… Common optical axis, 34 …… Filament image.

Claims (6)

[Claims] 1. A lighting device (14) for an overhead projector (10) comprising a truncated concave mirror (28), an incandescent filament (26) and a condenser lens (30), all of which are Arranged along a common optical axis (32) and the filament (26) is between the reflector (28) and the condenser lens (30) and the filament image (34) is the optical axis ( An overhead projector (10) arranged on top of (32) and between the filament (26) and the condenser lens (30)
Lighting equipment (14). 2. The illuminating device (14) according to claim 1, wherein the condenser lens (30) is a plano-convex glass lens. 3. The lighting device (14) according to claim 1, wherein the filament (26) is tightly wound and wound. 4. The illuminating device (14) according to claim 1, wherein the concave reflecting mirror (28) is spherical. 5. The illuminating device (14) according to claim 4, wherein the filament (26) has a center of curvature (36) of the spherical reflecting mirror (28) and the reflecting mirror (28). Lighting equipment in between (14). 6. The illumination device (14) according to claim 4, wherein the filament (26) and the image (34) are provided.
Is an illumination device between the center of curvature (36) of the spherical reflecting mirror (28) and the condenser lens (30).